The present invention relates to a digital demodulation system for use in a multi-level communication system and, more particularly, to an automatic gain control (AGC) circuit for controlling an input level of a multi-level discriminator to an optimum one and a transversal equalizer capable of exhibiting a sufficient equalizing ability.
While various kinds of microwave digital communication systems have been put to practical use, a predominant one is a high multi-level quadrature amplitude modulation (QAM) system. Although the high QAM system is effective to increase the amount of information that can be transmitted, it renders an apparatus design complicated and requires various circuits to be furnished with strict characteristics. One of such characteristics is the AGC characteristic which is set up in order that in a demodulator a demodulated multi-level signal may be controlled to an optimum input level of a multi-level discriminator. A construction elaborated to implement the strict AGC characteristic is disclosed in, for example, Japauese Unexamined Patent Publication No. 57-131152. However, the problem with the disclosed construction is that during the course of a pull-in a false or abnormal pull-in occurs from time to time although it may successfully control the input level of the discriminator to an optimum one. Assuming an 8-levels demodulated signal, for example, false pull-in occurs at levels which are deviated from a normal level by 7/5, 7/3, 7/9 and 7/11, preventing accurate main data signals from being recovered.
Meanwhile, a 16 QAM and other multi-level digital modulation systems are capable of transmitting information with high efficiencies but are quite susceptive to various distortions which are particular to transmission paths, thus severely suffering from fading. A transversal equalizer is an effective countermeasure against fading heretofore known in the art and is beginning to be regularly equipped with in high-efficiency communication systems. Generally, a transversal equalizer should preferably have such an equalizing characteristic that a limit equalizing ability under a condition wherein transmission distortions (intersymbol interference) are increased from relatively small ones and a limit equalizing ability under a condition wherein transmission distortions that are large enough to render the equalizer practically inoperable are sequentially reduced until the equalizer is restored to operable are equal to each other. At the present stage of technology, however, the limit equalizing ability in the second-mentioned condition is significantly small. This suggests that potential capability of a transversal equalizer is not fully exhibited.